1. Field of the Invention
The present invention relates to heavy dobbies used for forming the shed in weaving machines of very large width, of the type particularly employed for manufacturing tubular fabrics adapted for the production of endless dry felts or filters for the paper-making industry.
2. History of Related Art
Due to the very considerable amplitude of the stroke of the heddle frames associated with these dobbies, the conventional open-shed systems cannot be used and closed-shed dobbies must be employed in which each frame moves downwardly or upwardly from a mean position corresponding to the point of crossing of the assembly of the frames.
FIG. 1 of the accompanying drawings schematically shows the general arrangement of the heavy closed-shed dobbies of conventional type. Each of the heddle frames CL mounted on the weaving machine is connected by a system of rods 1 to a corresponding oscillating lever 2, articulated at 2a on a fixed frame. Opposite pivot point 2a. each lever 2, provided with a stop 2b. supports a double rocking hook 3 which is pivotally controlled by pushers of a reading mechanism of the dobby in order to come selectively into engagement with one or the other of two actuation knives 4, which, suitably guided horizontally, are controlled in order to simultaneously move reciprocally in opposite directions.
Actuation of the assembly of the double hooks 3 by the reading mechanism is effected when the two knives 4 lie in a central position in which the levers 2 are oriented vertically. It will be understood that, depending on whether a hook cooperates with one or the other of the knives 4, the corresponding frame CL moves vertically upwardly or downwardly from the position corresponding to the closure of the shed.
The mechanism for controlling the two knives 4 includes, at each of the opposite ends thereof, a connecting rod 5 articulated on one of the knives and on a balance beam 6 adapted to oscillate about a support pin 7. Each of the two balance beams 6 is equipped with two rollers 6a and 6b which roll on tracks 8a and 8b of a complementary cam 3 fitted on a shaft 8c. The shaft is driven in continuous rotation, via a transmission shown schematically at 9, by a motor of the weaving machine.
It will be readily understood that, although such a mechanism makes it possible to impart to the two knives the desired reciprocating movement, its operation at the high speeds now required of weaving machines becomes random, particularly due to the length of the kinematic chain, which includes a large number of articulating elements. Defects in weaving are, therefore, relatively frequent.
It is an object of the present invention to overcome this drawback.